The present invention relates generally to coated TiO.sub.2 particles for use as pigments, and particularly to TiO.sub.2 particles having densified silica coatings.
TiO.sub.2 has a high refractive index for its density, which renders it a superior pigment for use in coatings, e.g., paints. However, TiO.sub.2 is photoactive; exposure to ultraviolet radiation results in generation of free radicals on the surface of TiO.sub.2 particles. Thus, where a TiO.sub.2 pigment is employed in a paint exposed to sunlight, free radicals are generated which can migrate to the film-forming component of the paint, resulting in coating degradation or failure.
Therefore, minimizing free radical migration is desireable in order to provide lightfast and stable pigments for coating use. A widely-used approach to the problem of providing lightfastness involves coating TiO.sub.2 particles with a layer of densified silica. The following patent referenes disclose conventional silica coating densification techniques.
Werner, U.S. Pat. No. 3,437,502, discloses TiO.sub.2 pigment having high opacity and dispersibility, which is obtained by applying a dense silica coating to TiO.sub.2 followed by topcoating with alumina. The silica coating is applied by increasing the pH of a slurry of TiO.sub.2 particles to above 7. A predetermined quantity of sodium silicate is then added to the slurry, resulting in a rise in pH of the slurry to above 8, generally above 11. The pH is then gradually reduced over a period of several hours by addition of dilute acid, and the resulting silica-coated particles are cured at pH 6.0 to 7.5, at 60.degree. to 100.degree. C., for 30 minutes to several hours. The resulting product is then coated with alumina.
West, U.S. Pat. No. 4,125,412, discloses preparation of durable TiO.sub.2 pigments by slurrying silica-coated TiO.sub.2 particles at pH 9 to 10.5 while maintaining slurry temperature at 80.degree. to 100.degree. C.
European Patent Application No. 73,340 describes a similar method for coating TiO.sub.2 particles with amorphous dense silica prior to topcoating with alumina. Silica is applied by adding a soluble silicate to a TiO.sub.2 slurry at a temperature of at least 85.degree. C. and at a pH of from 9.8 to 10.1. The slurry is then neutralized in at least three neutralization steps by addition of acid.
Common to the teachings of the foregoing references is a requirement for relatively high temperatures and high alkalinity during the silica densification step. An extremely lightfast white pigment for use in paint systems can be made by depositing 4-6 weight percent amorphous dense silica on rutile TiO.sub.2 pigment. To coat the base TiO.sub.2 pigment with this quantity of dense silica requires several hours at 85.degree.-90.degree. C., during which mineral acids are used to precipitate silica from Na.sub.2 SiO.sub.3. This high temperature coating operation requires use of steel tanks, since less costly fiberglass tanks are not rated for use in the 85.degree.-90.degree. C. temperature range. Methods for providing dense coatings of more than 4 weight percent silica at lower temperatures, e.g., 65.degree.-80.degree. C., to allow use of fiberglass tanks, would be of significant interest to the TiO.sub.2 pigment industry.
It has now been found that codeposition of B.sub.2 O.sub.3 with SiO.sub.2 provides dense silica coatings at process temperatures which permit use of fiberglass slurry tanks. The boria-containing pigments resulting from this process are highly lightfast, and exhibit excellent gloss and dispersibility.